This application has priority benefit of Japanese Patent Application No. 00/320113, filed on Oct. 20, 2000.
(1) Field of the Invention
The present invention relates to a water based coating composition, a water based coating composition useful for coating the internal surface of a food can, and particularly a water based coating composition capable of forming a coating film showing a controlled eluation of bisphenol A.
(2) Description of Background Art
Recently, a water based coating composition is widely used in the art as a coating composition to be applied onto the internal surface of a can from the standpoints of working hygiene, environmental conservation and safety to fire. The water based coating composition applied onto the internal surface of the can may include ones containing mainly an esterification reaction product between an epoxy resin and a carboxyl group-containing acrylic resin as a resin component as disclosed, for example, in Japanese Patent Publication No. 41934/88 (U.S. Pat. No. 4,585,813 and U.S. Pat. No. 4,482,673) and Japanese Patent Publication No. 37026/84 (U.S. Pat. No. 4,458,040), Japanese Patent Application Laid-Open No. 329974/94, etc.
However, the above water based coating compositions use, AS A STARTING EPOXY RESIN, a high molecular weight epoxy resin obtained by reacting a low molecular weight epoxy resin with bisphenol A, resulting in that an unreacted bisphenol A remains in the coating composition. Recently, the bisphenol A causes troubles as an exogenous endcrine-disturbing chemical substance or an environmental hormone. Therefore, such a water based coating composition as to be free of bisphenol A remaining therein and as not to cause eluation of bisphenol A from a resulting coating film has been demanded. In connection therewith, the use of a bisphenols-undetected low molecular weight epoxy resin only as a starting epoxy resin may result a water based coating composition may form a coating film reduced in coating film performances such as coating workability, adhesion properties to a substrate, flavor properties and the like so as to be unsuitable as the coating composition applied onto the internal surface of the can. On the other hand, attempts to use an epoxy resin prepared by extracting and removing low molecular weight components including unreacted bisphenol A from the above high molecular weight epoxy resin as a starting epoxy resin have been made with the result that a satisfactory contact of the eluation of bisphenol A from the coating film has not been achieved.
It is an object of the present invention to provide a water based coating composition capable of forming a coating film not eluating bisphenol A therefrom without reducing coating film performances demanded on the coating film applied onto the internal surface of the can, for example, coating workability, adhesion properties, retorting resistant properties, mar resistance properties, hygienical properties, flavor properties and the like.
The present inventors made intensive studies to solve the above problems and percieved that the bisphenol A produced by thermal decomposition of the epoxy resin as a heat curing temperature of the coating film is increased may eluate from the coating film, resulting in finding out that a specified amount of the quaternary ammonium salt in the acrylic-modified epoxy resin makes it possible to form a coating film free of eluation of bisphenol A without reducing performances required for the coating film to complete the present invention.
That is, the present invention provides a water based coating composition prepared by reacting an epoxy resin (A) having a bisphenol A skeletal structure and a carboxyl group-containing acrylic resin in the presence of an amine compound to form an acrylic-modified epoxy resin (C), and neutralizing, and dispersing the acrylic-modified epoxy resin (C) into an aqueous medium, an amount of a quaternary ammonium salt in the acrylic-modified epoxy resin being in the range of 3.0xc3x9710xe2x88x924 mol or less per one gram of the resin (hereinafter may be referred to simply as 3.0xc3x9710xe2x88x924 mol/g or less).
In the composition of the present invention, the epoxy resin (A) used in the preparation of the acrylic-modified epoxy resin is an epoxy resin having a bisphenol A skeletal structure, and may include the following bisphenol A based epoxy resin (a), a composite bisphenol based epoxy resin (b) having a bisphenol A skeletal structure and bisphenol F skeletal structure in one molecule, and mixture of the above (a) and (b).
The bisphenol A based epoxy resin (a) may preferably include ones having a number average molecular weight in the range of 4,000 to 30,000, preferably 5,000 to 30,000, and an epoxy equivalent in the range of 2,000 to 10,000, preferably 2,500 to 10,000 from the standpoints of disversion stability in the aqueous medium, fabrication properties, hygienical properties and the like of the resulting coating film.
The bisphenol A based epoxy resin (a) may be prepared by one step polymerization method between bisphenol A and epichlorohydrin, or by two step polymerization method comprising adding bisphenol A to a bisphenol A based epoxy resin having a relatively low epoxy equivalent.
The relatively low epoxy equivalent bisphenol A based epoxy resin may include ones having an epoxy equivalent in the range of about 160 to about 2,000, and may include commercially available epoxy resins, for example, Epikote 828EL, Epikote 1001, Epikote 1004 and Epikote 1007 (trade names, marketed by Japan Epoxy Resin Co., Ltd. respectively); Araldite AER 250, Araldite AER 260, Araldite AER 6071, Araldite AER 6004 and Araldite AER 6007 (trade names, marketed by Asahi Kasei Epoxy Co., Ltd. respectively); Epomik R140, Epomik R301, Epomik E304 and Epomik R307 (trade names, marketed by Mitsui Chemicals, respectively); Adekaresin EP-4100 and Adekaresin EP-5100 (trade names, marketed by Asahi Denka Kogyo K.K., respectively); and the like.
The bisphenol A based epoxy resin (a) in the present invention may include commercially available ones such as Epikote 1010, Epikote 1256B40, Epikote 1256 (trade names, marketed by Japan Epoxy Resin Co., Ltd. respectively), and the like. The bisphenol A based epoxy resin (a) may also include a modified bisphenol A based epoxy resin prepared by modifying the bisphenol A based epoxy resin with a dibasic acid. The bisphenol A based epoxy resin to be reacted with the dibasic acid may preferably include ones having a number average molecular weight in the range of 2,000 to 8,000 and an epoxy equivalent in the range of 1,000 to 4,000. The dibasic acid may include a compound represented by the general formula: HOOCxe2x80x94(CH2)nxe2x80x94COOH where n is an integer of 1 to 12, specifically succinic acid, adipic acid, pimelic acid, azelaic acid, sebacic acid, dodecane diaid, hexahydrophthalic acid and the like, particularly adipic acid is preferable.
The modified bisphenol A based epoxy resin may be prepared by reacting a mixture of the bisphenol A based epoxy resin with the disbasic acid in the presence of an esterifying catalyst such as tri-n-butylamine and an organic solvent at a reaction temperature of 120 to 180xc2x0 C. for about 1 to 4 hours.
The modified bisphenol A based epoxy resin is such that the dibasic acid molecular chain acts as a plasticizing component and improve adhesion properties, resulting in that use of the modified epoxy resin as the bisphenol A based epoxy resin (a) is advantageous in improving fabricating properties and corrosion resistance of the resulting coating film.
The composite bisphenol based epoxy resin (b) is a bisphenol based epoxy resin having a bisphenol A skeletal structure and a bisphenol F skeletal structure in one molecule. In the present specification, the bisphenol A skeletal structure is represented by the following chemical formula (1), and the bisphenol F skeletal structure is represented by the following chemical formula (2). 
The composite bisphenol based epoxy resin (b) preferably has a number average molecular weight in the range of 4,000 to 30,000, preferably 5,000 to 30,000, and an epoxy equivalent in the range of 2,000 to 10,000, preferably 2,500 to 10,000 from the standpoints of dispersion stability in the aqueous medium, fabrication properties and hygienic properties of the resulting coating film.
The composite bisphenol based epoxy resin (b) may be a bisphenol based epoxy resin having the bisphenol A skeletal structure and the bisphenol F skeletal structure in one molecule, and a mixing ratio of the bisphenol A skeletal structure to the bisphenol F skeletal structure may not particularly be limited, but an equivalent ratio of bisphenol A skeletal structure/bisphenol F skeletal structure in one molecule is preferably in the range of 90/10 to 20/80 on an average.
The composite bisphenol based epoxy resin (b) may be prepared, for example, by combining at least one bisphenols selected from bisphenol A and bisphenol F and at least one glycidyl ethers selected from a relatively low epoxy equivalent bisphenol A based glycidyl ether and a relatively low epoxy equivalent bisphenol F based glycidyl ether so as to contain the bisphenol A skeletal structure and the bisphenol F skeletal structure, followed by subjecting to addition reaction.
The above bisphenol A, bisphenol F, the relatively low epoxy equivalent bisphenol A based glycidyl ether and the relatively low epoxy equivalent bisphenol F based glycidyl ether may be referred to as xe2x80x9cBis Axe2x80x9d, xe2x80x9cBis Fxe2x80x9d, xe2x80x9cA based Epxe2x80x9d and xe2x80x9cF based Epxe2x80x9d respectively.
The above combination in the preparation of the composite bisphenol based epoxy resin (b) may include, for example, (1) Bis F and A based Ep, (2) Bis A and F based Ep, (3) Bis F, F based Ep and A based Ep, (4) Bis A, F based Ep, A based Ep, (5) Bis F, Bis A and F based Ep, (6) Bis F, Bis A, A based Ep, and (7) Bis F, Bis A, F based Ep and A based Ep. A mixing ratio in the above combinations is in such an amount that the composite bisphenol based epoxy resin obtained by the addition reaction has the bisphenol A skeletal structure and the bisphenol F skeletal structure in one molecular.
The relatively low epoxy equivalent bisphenol A based glycidyl ether preferably include ones having an epoxy equivalent in the range of about 160 to about 2,000, preferably about 160 to about 1,000, and commercially available ones, for example, Epikote 828 EL, Epikote 1001, Epikote 1004 and Epikote 1007 (trade names, marketed by Japan Epoxy Resin Co., Ltd. respectively); Araldite AER 250, Araldite AER 260, Araldite AER 6071, Araldite AER 6004 and Araldite AER 6007 (trade names, marketed by Asahi Kasei Epoxy Co., Ltd. respectively); Epomik R140, Epomik R301, Epomik R304 and Epomik R307 (trade names, marketed by Mitsui Chemicals, respectively); A dekaresin EP-4100 and Adekaresin EP-5100 (trade names, marketed by Asahi Denka Kogyo K. K., respectively), and the like.
The relatively low epoxy equivalent bisphenol F based glycidyl ether may preferably include ones having an epoxy equivalent in the range of about 140 to about 2,000, preferably about 140 to about 1,000, and commercially available ones, for example, Epikote 807 and Epikote 806H (trade names, marketed by Japan Epoxy Resin Co., Ltd. respectively), Epomik R- 114 (trade name, marketed by Mitsui Chemicals), Adekaresin EP-4900 (trade name, marketed by Asahi Denka Kogyo K. K.), Epiclon 830 (s) (trade name, marketed by Dainippon Ink and Chemicals Inc.), Epototo YDF-170 (trade name, marketed by Tohto Kasei Co., Ltd.), and the like.
The composite bisphenol based epoxy resin (b) may include a modified epoxy resin modified with a dibasic acid, and the modified epoxy resin may be prepared, for example, by (1) a method which comprises modifying a bisphenol based epoxy resin having the bisphenol A skeletal structure and the bisphenol F skeletal structure in one molecule with a dibasic acid, (2) a method which comprises reacting a mixture of a relatively low epoxy equivalent bisphenol A based glycidyl ether (A based Ep) and a relatively low epoxy equivalent F based glycidyl ether (F based Ep) with a dibasic acid, and the like.
The bisphenol based epoxy resin having the bisphenol A skeletal structure and the bisphenol F skeletal structure in one molecule as in the above method (1) may preferably include epoxy resins obtained by the same preparation method as in the epoxy resin (b) except for non-modified composite bisphenol based epoxy resin (b) and a low molecular weight and having a number average molecular weight in the range of 2,000 to 8,000, and an epoxy equivalent in the range of 1,000 to 4,000. The dibasic acid used in the above methods (1) and (2) may include the same dibasic acid as used in the preparation of the modified bisphenol A based epoxy resin modified by the dibasic acid and included in the bisphenol A based epoxy resin (a).
In the above methods (1) and (2), a mixture of an epoxy component such as the bisphenol based epoxy resin or the glycidyl ether and dibasic acid may reacted in the presence of an esterifying catalyst such as tri-n-butylamine and an organic solvent at a reaction temperature of 120 to 180xc2x0 C. for about 1 to 4 hours to obtain a modified epoxy resin included in the composite bisphenol based epoxy resin (b).
The modified epoxy resin modified with the dibasic acid and included in the composite bisphenol based epoxy resin (b) is such that the dibasic acid molecular chain introduced into the molecule of the epoxy resin acts as a plasticizing agent so as to achieve improvement in adhesion properties, resulting in that the modified epoxy resin is advantageous in improving fablication properties and anticorrosive properties of the resulting coating film.
In the case where a mixture of the bisphenol A based epoxy resin (a) and the composite bisphenol based epoxy resin (b) is used as the epoxy resin (A) used in the preparation of the acrylic-modified epoxy resin, a mixing ratio of (a) to (b) is such that a solid content weight ratio of (a)/(b) is preferably in the range of 10/90 to 90/10, preferably 25/75 to 70/30 from the standpoints of retorting resistant properties, adhesion properties, anticorrosive resistance, etc. of the coating film.
The epoxy resin (A) may also include an epoxy resin prepared by subjecting the above exemplified epoxy resins as the epoxy resin (A) to extractionxe2x80xa2washing post treatment to remove the low molecular weight component including unreacted bisphenol A.
The carboxyl group-containing acrylic resin (B) (hereinafter may be referred to as acrylic resin (B)) used in the preparation of the acrylic-modified epoxy resin by reacting with the epoxy resin (A) is an acrylic copolymer containing, as the essential monomer component, a polymerizable unsaturated carboxylic acid such as acrylic acid, methacrylic acid, maleic acid, itaconic acid, fumaric acid and the like. The copolymer preferably has a weight average molecular weight in the range of 5,000 to 100,000, preferably 10,000 to 100,000, and a resin acid value in the range of 150 to 450 mg KOH/g, preferably 200 to 450 mg KOH/g from the standpoints of stability in the aqueous medium, and fabrication properties, retorting resisting properties, flavor properties and the like of the resulting coating film.
Monomer components other than the polymerizable unsaturated carboxylic acid used in the polymerization of the acrylic resin (B) may include, for example, C1-15 atkyl esters of acrylic acid or methacrylic acid such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate, 2-exhylhexyl (meth)acrylate, lauryl (meth)acrylate, benzyl (meth)acrylate, stearyl (meth)acrylte, cetyl (meth)acrylate and the like; cyclohexyl (meth)acrylate, isoburnyl (meth)acrylate; aromatic vinyl monomer such as styrene, xcex1-methylstyrene, vinyl toluene and the like; hydroxyl group-containing polymerizable unsaturated monomer including hydroxyalkyl (meth)acrylate such as 2-hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth) acrylate, hydroxyamyl (meth)acrylate, hydroxyhexyl (meth)acrylate and the like, and hydroxyl group-containing caprolactone-modified alkyl (meth)acrylate prepared by subjecting one mole of hydroxyalkyl (meth)acrylate such as hydroxyethyl (meth)acrylate and the like and 1 to 5 moles of xcex5-caprolactone to a ring opening addition reaction; acrylamide monomer such as acrylamide, methacrylamide, N-methoxymethyl (meth)acrylamide, N-ethoxymethyl (meth)acrylamide, N-n-propoxymethyl (meth)acrylamide, N-isopropoxymethyl (meth)acrylamide, N-n-butoxymethyl (meth)acrylamide, N-sec-butoxymethyl (meth)acrylamide, N-tert-butoxymethyl (meth)acrylamide, and the like; acrylonitride, methacrylonitride, vinyl acetate, ethylene, butadiene, and the like.
The acrylic resin (B) may be prepared by heating and copolymerizing a monomer mixture of the polymerizable unsaturated carboxylic acid and the other monomer component, for example, in an organic solvent and in the presence of a radical polymerization initiator or a chain transfer agent at 80 to 150xc2x0 C. for 1 to 10 hours. The polymerization initiator may include organic peroxide polymerization initiators such as benzoyl peroxide, t-butylperoxy-2-ethyl hexanoate, di-t-butyl peroxide, t-butylperoxybenzoato, t-amylperoxy-2-ethylhexanoate and the like, and azo polymerization initiators such as azobisisobutylonitrile, azobisdimethylvaleronitrile and the like. The chain transfer agent may include xcex1-methylstyrene dimer, mercaptan and the like.
The acrylic-modified epoxy resin (C) in the present invention is prepared by reacting the epoxy resin (A) and the carboxyl group-containing acrylic resin (B) in the presence of an amine compound.
The reaction between the epoxy resin (A) and the carboxyl group-containing acrylic resin (B) may be carried out by heating and esterifying in an organic solvent, for example, and in the presence of a tertiary amine compound such as triethylamine, dimethyl ethanolamine, triethanolamine, monomethyl diethanolamine, morpholine an the like as the esterifying catalyst at 80 to 120xc2x0 C. for 0.5 to 8 hours to obtain an acrylic-modified epoxy resin (C). A mixing amount of the amine compound is in the range of 1 to 10% by weight based on a total solid content of the resins (A) and (B). A mixing amount more than the above range undesirably increase a proportion of a quaternary ammonium salt-forming reaction on the esterification reaction.
A mixing ratio of the epoxy resin (A) to the acrylic resin (B) may be arbitrarily be selected depending on coating workability and coating film performances, but a solid content weight ratio of (A)/(B) is in the range of 10/90 to 95/5, preferably 60/40 to 90/10.
The acrylic-modified epoxy resin (C) obtained by the esterification reaction preferably has an acid value in the range of 20 to 120 mg KOH/g, preferably 30 to 100 mg KOH/g. An acid value less than 20 mg KOH/g makes impossible to obtain dispersion stability in the aqueous medium. On the other hand, an acid value more than 120 mg KOH/g raises problems of eluation of bisphenol A from the resulting coating film, and of undesirable water resistance. Preferably, the acrylic-modified epoxy resin (C) essentially has no epoxy group from the standpoint of storage stability.
The acrylic-modified epoxy resin (C) is neutralized and dispersed into an aqueous medium, and a neutralizing agent used for the above neutralization may preferably include amines and ammonia.
Typical examples of the amines may include triethylamine, triethanolamine, dimethyl ethanolamine, diethyl ethanolamine, morpholine and the like. Of these, triethylamine and dimethyl ethanolamine are preferable. A degree of neutralization of the acrylic-modified epoxy resin is in the range of 0.2 to 2.0 equivalent of neutralization relative to carboxyl group in the resin.
In the present invention, an amount of a quaternary ammonium salt formed during the esterification reaction and neutralization and contained in the acrylic-modified epoxy resin is essentially controlled in the range of 3.0xc3x9710xe2x88x924 mol or less, preferably 2.5xc3x971031 4 mol per one gram of the resin. An amount of the quaternary ammonium salt more than 3.0xc3x9710xe2x88x924 mol per one gram of the resin undesirably raises problems of eluation of bisphenol A from the resulting coating film. Measurement of the amount of the quaternary ammonium salt is carried out by an electrical conductivity titration method as disclosed in Japanese Patent Application Laid-Open No. 160029/96. That is, an indicator solution prepared by dissolving an indicator having sulfonic group and hydroxyl group as functional groups into a solvent is dropped into a sample solution prepared by dissolving a sample after starting a reaction into a solvent to carry out a titration reaction, followed by plotting a relationship between a titer and an electrical conductivity for a first titration reaction step of reacting the indicator and a quaternary ammonium saltized epoxy compound to form both sulfonic group and hydroxyl group-ionized indicator and carboxylic acid, and for a second titration reaction step of reacting the indicator and the ionized indicator to form only sulfonic group-ionized indicator respectively, determining a titer t1 in the first titration reaction step from a titer at an intersecting point between a straight line along plots in the first titration reaction step and a straight line along plots in the second titration reaction step, and determining an amount (mol/g) of quaternary ammonium salt per one gram of a solid content of the sample in accordance with the following formula (1):
Amount of Quaternary
ammonium salt (mol/g)=t1 (ml)xc3x972xc3x97concentration of indicator (mol/1)xc3x97({fraction (1/1000)})xc3x97{100/(sample (g)xc3x97solid content (%))xe2x80x83xe2x80x83(1). 
The aqueous medium, into which the acrylic-modified epoxy resin is dispersed, may include water and a mixture of water with an organic solvent. The organic solvent may include any known organic solvents miscrible with water and capable of keeping stability in the aqueous medium of the acrylic-modified epoxy resin. The organic solvent may include, for example, alcohol solvent, cellosolve solvent, carbitol solvent and the like. Specific examples of the organic solvent may include alcohol solvent such as n-butanol; cellosolve solvent such as ethylene glycol monobutyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monomethyl ether, propylene glycol monomethyl ether and the like; carbitol solvent such as diethylene glycol monoethyl ether and the like, and the like. Other inactive organic solvents immiscible with water may also be used as the above organic solvent in such an amount as not to impair stability in the aqueous medium of the acrylic-modified epoxy resin, and may include, for example, aromatic hydrocarbons such as toluene, xylene and the like, esters such as ethyl acetate, butyl acetate and the like, and ketones such as methyl ethyl ketone and the like. An amount of the organic solvent in the water based coating composition of the present invention may be so as to be in the range of 50% by weight or less in the aqueous medium from the standpoint of an environmental protection.
A method of neutralizing and dispersing the acrylic-modified epoxy resin (C) into the aqueous medium may include, for example, a method which comprises slowly adding the acrylic-modified epoxy resin to an aqueous medium containing a neutralizing agent with agitation, a method which comprises neutralizing the acrylic-modified epoxy resin (C) with a neutralizing agent, followed by adding an aqueous medium to a neutralized product or by adding the neutralized product to the aqueous medium, and the like.
The water based coating composition of the present invention may also be prepared further by subjecting a polymerizable unsaturated monomer to seed polymerization in an aqueous dispersion obtained by the neutralization of the acrylic-modified epoxy resin (C).
The polymerizable unsaturated monomer used in the seed polymerization may arbitrarily be selected from examples of the polymerizable unsaturated carboxylic acid and other monomer components as used in the polymerization of the acrylic resin (B), preferably include styrene and C1-24 alkyl esters of acrylic acid, more preferably combinations of styrene and alkyl esters having 4 or more carbon atoms of acrylic acid in that an interparticle polymerization in the water dispersion can be sufficiently performed, and that a coating film having good film performances can be formed. Polymerization conditions of temperature and time in seed polymerization may vary depending on kinds of monomers and initiators, but is such that the reaction temperature is in the range of about 30 to 100xc2x0 C., and the reaction time is in the range of 30 minutes to 10 hours. A solid content of the resin dispersion obtained by seed polymerization may not particularly be limited, but usually is in the range 10 to 70% by weight.
A mixing amount of the polymerizable unsaturated monomer is suitably in the range of 2 to 50 parts by weight per 100 parts by weight of the solid content of the acrylic-modified epoxy resin (C).
The initiator used in the seed polymerization may preferably include a redox catalyst, specifically a combination of a reducing agent such as ammonium bisulfite and an oxidizing agent such as t-butylhydroperoxide.
In addition to the water based coating composition prepared by neutralizing and dispersing the acrylic-modified epoxy resin (C) into the aqueous medium, and the water based coating composition prepared by subjecting the polymerizable unsaturated monomer to seed polymerization in a water dispersion obtained by neutralizing the acrylic-modified epoxy resin (C), the water based coating composition of the present invention may also include, in addition to the above water based coating compositions, may optionally contain curing agents such as resol based phenol resin, novolak based phenol resin, melamine resin, benzoguanamine resin and the like, surface active agents, waxes, anti-foaming agents, pigments, perfumes and the like.
The water based coating composition of the present invention may be applied onto various kinds of coating substrates, for example, untreated or surface treated metal plates such as aluminum plate, tin-free steel plate, tinplate and the like; precoated metal plates prepared by applying a primer coating such as epoxy based primer coating, vinyl based primer coating and the like to the above metal plates respectively; molded metal plates prepared by fabricating the above metal plates and precoated metal plates in the form of a can or the like, and the like.
The method of coating the water based coating composition of the present invention may include any known methods, for example, roll coater coating, spray coating, dip coating, electrodeposition coating and the like. Of these, the spray coating is preferable. A coating film thickness from the water based coating composition of the present invention may arbitrarily be selected depending on uses, but is such that a dry film thickness is in the range of 3 to 20 xcexcm. Drying conditions of the coating film are such that a substrate-reaching maximum temperature is in the range of 150 to 300xc2x0 C. and a drying time is in the range of 5 seconds to 30 minutes, preferably 200 to 280xc2x0 C. and 10 seconds to 2 minutes.
Increase of the amount of bisphenol A eluated from the coating film with increase of the heat curing temperature of the coating film may be caused by the thermal decomposition of the epoxy resin, and consequently it is guessed that the quaternary ammonium salt or carboxylate ion may act as a catalyst on the thermal decomposition of the epoxy resin.
Thus, according to the water based coating composition of the present invention, control of the amount of the quaternary ammonium salt in the acrylic-modified epoxy resin (C) within a specified range makes it possible to form a coating film not eluating bisphenol A without reducing coating film performances demanded on the coating film applied onto the internal surface of a can.